test packaged host cargo runtime with an empty PATH

flake.nix

@@ -21,9 +21,11 @@
           targets = [ "wasm32-wasip2" ];
           extensions = [ "rust-src" ];
         };
-        # Crane builds the dependencies once (`cargoArtifacts`, keyed only on
+        # The runtime cargo actor needs Cargo, rustc (with wasm32-wasip2), and
-        # Cargo.toml/Cargo.lock), so a change to our own crates rebuilds
+        # the native linker, even when the host itself is started with a bare
-        # just them.
+        # PATH. Keep this same runtime path in the packaged host and exercise
+        # the installed binary below with PATH deliberately empty.
+        runtimePath = pkgs.lib.makeBinPath [ rust pkgs.stdenv.cc ];
         craneLib = (crane.mkLib pkgs).overrideToolchain rust;
         common = {
           pname = "sausage";
@@ -47,7 +49,7 @@
           nativeBuildInputs = [ pkgs.makeWrapper ];
           # The `cargo` actor runs cargo at runtime. Build scripts need a C linker.
           postInstall = ''
-            wrapProgram $out/bin/sausage --prefix PATH : ${pkgs.lib.makeBinPath [ rust pkgs.stdenv.cc ]}
+            wrapProgram $out/bin/sausage --prefix PATH : ${runtimePath}
           '';
           meta.mainProgram = "sausage";
         });
@@ -91,6 +93,67 @@
             rm -f cli/tests/actors/toolbox/Cargo.lock
           '';
         });
+        # Exercise the installed host exactly as the loop launches it: the
+        # caller has no Cargo on PATH, so only the packaged runtime toolchain
+        # can run the native tests and the wasm32-wasip2 build.
+        checks.runtime-toolchain = pkgs.runCommand "sausage-runtime-toolchain" {
+          nativeBuildInputs = [ pkgs.expect ];
+        } ''
+          set -eu
+          root="$TMPDIR/runtime-toolchain"
+          world="$root/world"
+          emptyPath="$root/empty-bin"
+          mkdir -p "$emptyPath" "$root/home" "$root/cargo-home"
+          for actor in broker console cargo fs; do
+            mkdir -p "$world/actor/$actor/public" "$world/actor/$actor/data"
+          done
+          source="$world/actor/console/public/probe"
+          mkdir -p "$source/src" "$source/tests"
+          cat > "$source/Cargo.toml" <<'TOML'
+          [package]
+          name = "runtime-probe"
+          version = "0.1.0"
+          edition = "2024"
+          TOML
+          printf 'fn main() {}\n' > "$source/src/main.rs"
+          cat > "$source/tests/probe.rs" <<'RUST'
+          #[test]
+          fn native_test_harness_runs() {
+              assert_eq!(2 + 2, 4);
+          }
+          RUST
+          export SAUSAGE_BIN=${sausage}/bin/sausage
+          export WORLD="$world"
+          export EMPTY_PATH="$emptyPath"
+          expect <<'EOF'
+          log_user 1
+          set timeout 300
+          set env(PATH) $::env(EMPTY_PATH)
+          set env(HOME) $::env(WORLD)
+          set env(TMPDIR) [file dirname $::env(WORLD)]
+          set env(CARGO_HOME) [file join [file dirname $::env(WORLD)] cargo-home]
+          spawn $::env(SAUSAGE_BIN) run $::env(WORLD)
+          send -- "@cargo {\"type\":\"build\",\"source\":\"/actor/console/public/probe\"}\r"
+          expect {
+              -re {"type":"build_failed"} {
+                  puts stderr "packaged cargo actor build failed with an empty caller PATH"
+                  exit 1
+              }
+              -re {"type":"built"} {
+                  expect {
+                      -re {"passed":1} { send -- "/quit\r" }
+                      timeout { puts stderr "native cargo test did not pass"; exit 1 }
+                  }
+              }
+              timeout { puts stderr "packaged host did not build the probe"; exit 1 }
+          }
+          expect eof
+          set result [wait]
+          if {[lindex $result 3] != 0} { exit [lindex $result 3] }
+          EOF
+          mkdir -p "$out"
+          touch "$out/passed"
+        '';
 
         apps.default = { type = "app"; program = "${sausage}/bin/sausage"; };
         apps.loop = { type = "app"; program = "${loop}/bin/sausage-loop"; };